That's a bold statement.. This is quite a point of contention. We have human settlements in Chile that date no more recently than 12,500 B.P. and quite good evidence for settlements 33,000 B.P. The evidence for pre-Clovis settlements however is no longer disputed by the majority of anthropologists studying paleoamerican origins. Since sabre-cats and ground sloths were in South America 8,000 B.P. how does this provide evidence for anthropomorphic extinction when we have irrefutable evidence for human settlements dating to 12,500 B.P and strong evidence for human settlements going back as far as 33,000 B.P.? I disagree with your statement that "Pleistocene/Holocene hunter gatherers had a more pronounced effect on ecology than we've tended to give them credit for". This is conjecture of a high order, we simply don't know how great or small their effect was. In the last 25 years the realization of anthropomorphic climate change seems to have shifted thinking to giving primacy to anthropologic causes in other areas.
John Thornton